Numerous methods are known for determining optically, therefore without contact, the various characteristics of a surface. One such method is known, for example, from French patent FR-A-2 484 633 and French patent FR-A-2 172 213. Devices are also known for measuring the roughness and relief of a surface, for instance from French patent FR-A-2 208 107 or FR-A-2 593 599. These methods generally employ collimated light beams directed onto the surface and rediffused from the surface under conditions which permit obtaining local surface data; the set of local data permits the reconstitution of general data for the surface.
However, as concerns the surface finish of metal parts a person skilled in the art has until present relied above all upon his own experience and empirical judgement, which the above-mentioned methods, overly sophisticated, are incapable of replacing. A person skilled in the art relies upon the visual aspect of the metal examined in order to attribute a classification: although the classification is not unrelated to the roughness of the surface it is not the knowledge of the latter which determines the classification, but rather criteria such as homogeneity, "colour" or "density" (an open or tight aspect) of the sample, these criteria defining rather the texture of the surface than its roughness.